Subtract.
Explanation: First, let's start at ${17}$ on the number line. $0$ $5$ $10$ $15$ $20$ $\boxed{17}$ Next, to subtract we move left ${15}$ on the number line. $0$ $5$ $10$ $15$ $20$ $17$ $- 15$ The jump ends at $2$. $0$ $5$ $10$ $15$ $20$ $17$ $- 15$ $\boxed{2}$ ${2}={17} - {15}$